A brushless DC motor requires electronic commutation of its armature currents. This is typically accomplished, for a trapezoidal back EMF, three phase motor, by means of a bridge switching circuit containing six semiconductor switches, as shown in FIG. 3 and 4. When a predetermined upper switch and non-series lower switch are simultaneously conducting, an armature current flows through two of the three phase windings in series to electromagnetically interact with the permanent magnet rotor and develop torque in a predetermined direction. Commutation to a different pair of armature windings is accomplished by turning off one of the switches and turning on a different non-series switch of the same level (upper or lower), with a control providing switching signals to the switch gates in the proper sequence and with the proper timing. Motor operation is thus controlled by activating the switches in a predetermined pattern.
The electric currents in the activated phases are controlled, usually by pulse width modulation at a higher frequency, to control motor torque. This modulation, in combination with the inductance of the windings, produces an average motor current, and thus a smooth motor torque, except during commutation. In traditional commutation, where the phases are turned off and on abruptly, the rise and fall of the phase currents are controlled only by the network properties of the motor and switching circuitry. The forcing function for the phase turning off is not the same as it is for the phase turning on, and one of the currents (rising or falling) will change faster than the other. The unequal current in the phase turning off and the phase turning on can produce a disturbance in the average motor current, and thus a variation in torque, at each commutation event; and such disturbances can drive the motor structure to generate audible noise. In some applications of such motors these noises may be objectionable. In particular, use of such a control and motor in an electric power steering system in modes of operation producing high phase currents at low motor speed can produce an annoying clicking noise at commutation, which may be termed "zipper noise." It is an object of this invention to control the phase currents of such a motor during commutation to reduce or eliminate such objectionable noise.